As described in U.S. Pat. Nos. 2,297,691, 2,357,809, etc., the electrophotographic process in the prior art comprises uniformly charging a photoconductive insulation layer, exposing the layer, eliminating electric charges on the exposed area thereby forming electric latent images, visualizing the latent images by depositing electrically charged fine powder referred to as toners to the latent images (developing step), transferring the thus obtained visual images to transfer material such as transfer paper (transfer step) and, thereafter, permanently fixing them by heating, pressurizing or other like adequate fixing process (fixing step).
In this way, toners have to provide functions required not only in the developing step but in each of the transferring and fixing steps.
Furthermore, as the photosensitive material for copying machines, printers, etc. on adopting electrophotographic process, those using organic photosensitive materials in addition to conventional inorganic materials (amorphous selenium alloy, cadmium sulfide, etc.) have been increased in recent years. This is due to easy manufacture, reduced cost, non-toxicity, etc. of the organic photosensitive material.
Most of the organic photosensitive materials require in view of their properties those toners having a positively charged property. Furthermore, development of positively charged toners at high performance has keenly been demanded due to the increased use of laser beam printers or the like that conduct reversing development although the inorganic photosensitive materials are used.
A toner comprising a pigment, a charge-controlling agent and as a binder a vinyl polymer such as a copolymer of styrene and acrylic acid or an epoxy resin has been used. This binder is neutral in view of the electrical charging and it is easy to produce. Those toners using the resins as the binder and capable of satisfying all of important properties for the toners, that is, anti-offset property, blocking resistance, low temperature fixing property, anti-vinyl chloride migration, etc. have not yet been obtained in view of the nature of the resins employed.
That is, in the case of the vinyl polymer represented by the styrene-acryl copolymer, if it is intended to improve the anti-offset property, the softening point and the crosslinking density of the resin have to be increased, at the expense of the low temperature fixing property. On the contrary, if the improvement in the low temperature fixing property is intended, a difficulty arises in the anti-offset property and the blocking resistance. Further, since the solubility parameter of the styrene-acryl copolymer is nearly equal with the solubility parameter of a plasticizer such as dioctyl phthalate contained in vinyl chloride, the plasticizer migrates toward the toners thereby causing contamination of copy images reserved in vinyl chloride sheets and the like.
In the case of using an epoxy resin, although the vinyl chloride migration resistance can be provided, it is very difficult to improve the anti-offset property and low temperature fixing property by the resin alone and, particularly, it can not be used in a device using a heat roll type fixing machine.
Although, improvement has been hitherto attempted for these drawbacks, no solution has yet been obtained.
While on the other hand, polyesters have been noted as a binder resin having a wide molecular weight distribution and capable of satisfying required characteristics such as anti-offset property, low temperature fixing property, blocking resistance and vinyl chloride migration resistance in a well balanced manner. In this case, it has been known that the relationship between the charging property and the acid value of a polyester is substantially in a proportional relationship, that is, the higher the acid value the greater the negative charging property of the resin. Polyesters as described in Japanese Patent Application Laid-Open Nos. Sho 57-37353, 57-109825, etc. are excellent binder resin having crosslinked structures and sufficient negatively charged property without using the charge controller due to their relatively high acid values.
However, if these cross-linked type polyesters are intended to be used as the binder resin for toners having a positively charged property, countermeasures such as the blending of a great amount of positive charge controller are necessary in order to provide the positively charged property, because of the intense negatively charged property of the resin per se. Generally, although a nigrosine dye or the like has been known as a positive charge controller, if the dye is used in a great amount, it shows a poor compatibility with the binder resin constituting the main ingredient of the toners. Thus, if the toner particles are mixed for a long period of time in a developing device, the destruction of the toner particles occur and, in a case where the nigrosine or the like is merely dispersed, particles of adverse (negative) polarity not containing nigrosine are formed thereby causing a so-called background fogging in which toners are deposited to the area where there are no image signals. While on the other hand, if a great amount of the nigrosine dye or the like is used, the amount of charge varies depending on the circumstantial humidity due to the hydrophilic property to degrade the image quality. Further, the nigrosine type dye involves another drawback that it generally shows intense coloration and is not suitable to color toners.
While on the other hand, as copying machines have been used more and more generally, they are often installed under severe conditions. For example, those conditions such as high temperature and high humidity or low temperature and low humidity. Then, it is indispensable that clear images should also be obtained in such severe conditions just as those under normal circumstances. However, although the toners have favorable electric properties such as desirable charging amount and insulation resistance under usual conditions, they are greatly varied under high temperature and high humidity or low temperature and low humidity conditions to often result in the degradation of the images. For example, the image density is reduced under low temperature, low humidity condition, whereas the image density is increased under high temperature and high humidity condition and in an extreme case, almost all of the evenly black portion becomes blank. Those polyesters having hydrophilic carboxyl groups and hydroxyl groups at the terminal ends of the polymer may some time readily suffer from the circumstantial effects as described above if the concentration, that is, the acid value or the hydroxyl value thereof is too high.
Accordingly, the above mentioned conditions have been required for those toners capable of obtaining clear images under all sort of circumstances quite in the same manner as those under the usual circumstantial conditions.